I. Introduction
This invention relates to reduction of reflection from a substrate surface during the processing of photoresists in integrated circuit manufacture. More particularly, this invention relates to antireflective coatings (ARC's) and a simplified process for use and removal of the same to provide an image of enhanced resolution.
II. Description of the Prior Art
It is known in the art that during the manufacture of integrated circuits, silicon wafers are coated with photoresist, exposed to activating radiation, and developed to define a relief image over the wafer surface. The relief image defines open areas over the substrate in a desired image pattern to be transferred to a substrate. The image is transferred to the surface of the substrate by surface modification of the substrate in a negative image of the pattern within the photoresist coating, such as by removal of a portion of the substrate surface by an etching process or by implantation of an atomic species into the substrate surface. During these processes, the coating of the photoresist in the image pattern functions as a protective mask to prevent surface modification of the substrate underlying the photoresist mask. The resolution of the image transferred to the substrate is dependent upon the resolution within the imaged photoresist coating.
There are factors in addition to the resolution capability of the photoresist used that influence the quality or resolution of the image transferred to a photoresist masked substrate. For example, with reflective integrated circuit substrates, such as aluminum, exposure of a photoresist coating causes reflection of diffused activating radiation (light) from the integrated circuit substrate back into the photoresist coating. Standard photoresists are susceptible to surface reflections which degrade the fine-line images required for integrated circuit manufacture. This degradation occurs due to reflection of diffused light from the integrated circuit substrate back into the photoresist layer resulting in exposure of the photoresist layer in areas where imaging is not desired.
To prevent reflection of activating radiation into a photoresist coating, it is known to provide antireflective layers (ARC's) between a substrate and a photoresist layer. These antireflective layers typically comprise an adsorbing dye dispersed in a polymer binder though some polymers contain sufficient chromophores whereby a dye is not required. When used, the dye is selected to adsorb and attenuate radiation at that wavelength used to expose the photoresist layer thus reducing the incidence of radiation reflected back into the photoresist layer. During the conventional processing of an integrated circuit substrate coated with the combination of an antireflective layer and a photoresist layer, the photoresist is exposed to activating radiation and developed to form a relief image, i.e., portions of the photoresist layer are removed by development with a liquid developer and portions remain as a mask defining a desired pattern. To alter the underlying substrate, the antireflective layer must be removed to bare the substrate in a desired image. Removal of the antireflective layer may be by dissolution with a liquid that simultaneously dissolves both the photoresist and the antireflective layer or by dry etching such as with an oxygen plasma.
In those processes using solely wet development, the developer used to develop the imaged photoresist layer also dissolves the underlying antireflective layer. However, simultaneous development of the photoresist coating and the underlying antireflective coating often leads to undercutting of the imaged photoresist coating caused by the developer seeping beneath those portions of the photoresist coating which are insoluble in developer. The seepage causes dissolution of the antireflective coating beneath the photoresist layer and partial lift-off of the photoresist coating at the relief image margins resulting in a loss of fine line image resolution.
To avoid undercutting of the antireflective coating during development of the photoresist coating, dry etching of the antireflective coating such as with an oxygen plasma has been used. In this process, the antireflective coating used is one that is insoluble in developer for the imaged photoresist. Therefore, contact of the imaged photoresist coating with the developer does not dissolve the underlying antireflective coating. Following development, the structure formed consists of the substrate coated with the antireflective layer overcoated with a photoresist in a desired relief image pattern which functions as a mask over the antireflective layer. Following development of the photoresist coating, the antireflective coating bared by removal of the photoresist is removed by dry etching the entire surface of the coated wafer, typically with an oxygen plasma to ash the antireflective layer and thereby form the desired relief image over the integrated circuit substrate. Details for plasma etching can be found in Elliott, Integrated Circuit Fabrication Technology, McGraw Hill Book Company, 1982, pp 259 to 308, incorporated herein by reference.
The process utilizing dry etching also suffers several disadvantages. One disadvantage is that the step of dry etching is an additional step requiring special equipment that adds to the overall processing sequence time and cost for the fabrication of the integrated circuit. Moreover, plasma etching is a blanket etching step whereby the photoresist mask and the antireflective coating are both exposed to the plasma. This often leads to degradation of the overlying photoresist coating resulting in an uneven and partially removed photoresist coating. Nonuniformity of the coating caused by a dry etch step increases variations in the delineated microcircuitry sizing following the etch. Reduction in the thickness of the mask comprising the photoresist and underlying delineated antireflective layer decreases the thickness of the coating available for masking the substrate.
In addition to difficulties encountered with removal of the antireflective coating, other problems are often encountered when an antireflective coating is used in combination with a photoresist coating. One such problem is that of carefully selecting an antireflective coating that is compatible with the photoresist used. The antireflective coating should be inert with respect to the photoresist coating while firmly bonding to the coating. It is desirable that the antireflective coating be chemically inert to avoid photoresist contamination caused by migration of chemical species from the antireflective coating into the photoresist coating that could alter or degrade the response of the photoresist to activating radiation and development. At the same time, it is necessary that the antireflective coating firmly bond to the photoresist coating to avoid lift-off of the photoresist coating during processing of the underlying substrate.
It is therefore an object of this invention to provide a process for preventing reflection of activating radiation by use of an antireflective coating where removal of the antireflective coating is by a process other than by dry etching or simultaneous wet development of both a photoresist layer and the antireflective layer.
A further object of this invention is to decrease reflection of light into a photoresist layer by use of a process whereby the thickness of an antireflective layer may be increased to make the same more opaque.
A still further object is to reduce the process cycle time for the manufacture of integrated circuits using antireflective coatings.
An additional object of this invention is to increase adhesion between an antireflective coating and a photoresist mask.
A primary object of this invention is to preserve the fine lines desired in an exposed and developed photoresist mask and in an image transferred to an underlying substrate.
Other objects and advantages will become apparent from the following more complete description and claims.